bhyve nvme: Fix firmware read only initialization

Summary:
Code was using the mask value without the shift.

Test Plan: Within FreeBSD/Linux guest, Identify NVMe controller to check the result.

Reviewed by: chuck, imp
MFC after: 2 weeks
Signed-off-by: Wanpeng Qian <wanpengqian@gmail.com>
Differential Revision: https://reviews.freebsd.org/D32659